1. Field of the Invention
The present invention relates generally to a system for rapidly causing a temperature rise in a battery of a hybrid vehicle or an electric car, and more particularly to the system for rapidly causing a temperature rise in the battery while assuring the safety of the battery and the system for rapidly causing a temperature rise in the battery while keeping the voltage of electric power, applied from the battery to an electric motor, at a value required to drive the vehicle or car at a desired speed.
2. Description of Related Art
A voltage converting device mounted on a hybrid vehicle or an electric car rapidly increases the temperature of a rechargeable secondary battery. This device is, for example, disclosed in Published Japanese Patent First Publication No. JP2005-312160.
In this device of the Publication, a temperature sensor detects a temperature of a direct-current (dc) power source. When the source temperature is lower than a reference value, the device performs a temperature increasing control for the source. More specifically, a control unit controls a boost converter to repeatedly perform a voltage increasing operation and a voltage dropping operation. In the voltage increasing operation, electric power is sent from the power source to a capacitor through resistors of the power source. In the voltage dropping operation, electric power is sent from the capacitor to the power source through the resistors. Therefore, the temperature of the source heated by the resistors is increased. When the source temperature exceeds the reference value, the temperature increasing control is stopped, and the control unit controls the boost converter to perform a normal operation. In this normal operation, the voltage increasing operation, the voltage dropping operation or repetition of the increasing and dropping operations is performed. The voltage of the electric power accumulated in the capacitor is applied to the motor as a driving voltage, required to drive the motor at a desired speed.
However, when a rechargeable battery such as a lithium-ion rechargeable battery or the like is used as the dc power source, the internal resistance of the battery is increased during a low temperature period of time in which the atmospheric temperature is, for example, lower than the freezing point. Therefore, when large electric current flows through the battery at a low frequency during the voltage rising and dropping operations repeatedly performed, a change in the voltage of the battery is increased. In this case, the battery voltage sometimes exceeds its upper limit or becomes lower than its lower limit. Therefore, it is difficult to rapidly increase the battery temperature while assuring the safety of the battery (first drawback of the voltage converting device).
For example, in the case of the lithium-ion rechargeable battery, when the battery is overcharged so as to exceed the upper limit of the battery voltage, electrolytic solution of the battery is sometimes oxidized near the positive electrode of the battery, the crystal structure of the positive electrode is sometimes broken or damaged, and lithium is sometimes deposited on the negative electrode of the battery. In contrast, when the battery is over-discharged so as to become lower than the lower limit of the battery voltage, copper composing the negative electrode is sometimes dissolved away by the electrolyte. In the same manner, any rechargeable battery other than the lithium-ion rechargeable battery is also damaged when being overcharged or over-discharged.
Further, the driving voltage applied from the capacitor to the motor is changeable. When the engine of the vehicle or car starts driving or when the motor is operated to run the vehicle or car at a high speed, this driving voltage is sometimes increased to the maximum value such as 650 V. In this situation, when the temperature increasing control is performed, the voltage of the capacitor is considerably decreased during the voltage drop operation to be lower than the required driving voltage, and a voltage lower than the driving voltage, which is insufficient to drive the motor, is undesirably applied to the motor. Therefore, it is difficult to generate a driving torque, required to start the running of the vehicle or car or to run the vehicle or car at a desired speed, in the motor (second drawback of the voltage converting device).